Ensure you get a tight seal on your growler or jug with these 38mm Black Plastic Screw Caps. Each cap features a conical polyethylene liner to provide a wedge-type seal both across the top of the container and the inside diameter of the opening. The seal helps to keep the contents fresh and provides better pressure retention than standard metal caps for carbonated beverages. These caps work great on our 64 and 32 ounce glass growlers, as well as many gallon jugs.